Heretofore, variable speed drives have normally included a drive motor which operated at a constant output speed. Positioned on the output shaft of the drive motor was a pair of disks which were axially displaceable. An output drive shaft was spaced from the output shaft of the motor and had another pair of axially displaceable disks provided thereon. A belt extended between the pair of disks carried on the output shaft of the drive motor and the disks carried on the output shaft of the variable speed drive. In order to vary the coupling between the output shaft of the motor and the output shaft of the variable speed drive, a cam arrangement was used for axially displacing one of the disks carried on the output shaft of the motor. This in turn caused the belt extending between the two pair of disks to be radially displaced so as to vary the output speed of the output shaft of the variable speed drive. In one particular application such as being manufactured by Reliance Electric and referred to as Reeves XV Motor Drive, a hand wheel was coupled through gears to the cam for rotating the cam in order to displace the axially displaceable disk carried on the output shaft in order to adjust the output speed of the variable speed drive.
Automatic closed loop drive systems have also been used in the past and one such device is manufactured by Reliance Electric and is referred to as an ERC Package Control System. In that particular device, there is a magnetic pulse pickup for sensing the RPM of the output shaft of the variable speed drive. These pulses indicating the RPM of the output shaft of the variable speed drive are fed to a transmitter which in turn transmits a signal to a customer computer control center. At the customers' computer control center, a signal indicating the desired output speed of the variable speed device is generated and fed to an amplifier which is used for controlling an electronic remote control (ERC) for the variable speed drive. The ERC is equipped with a control motor that by rotating in one direction makes the output shaft of the variable speed drive go faster and rotating in the other direction go slower. A potentiometer is physically connected to the control motor shaft that is used for adjusting the output speed of the variable speed drive and generates a signal indicating the physical position of the arm. The potentiometer in turn is connected to the amplifier for making correction signals for the variable speed drive.
Another drive system is manufactured by T. B. Woods Sons Company of Chambersburg, Pa. In the Woods' device, there is a control motor associated with the variable speed drive for physically moving the input drive motor relative to the output drive shaft of the variable speed device for varying the output speed of the variable speed drive. The control motor is connected to a screw which upon being rotated physically moves the input drive motor back and forth to vary the output speed of the drive. A correction signal is produced by a circuit for driving the control motor for physically shifting the coupling between the input drive motor of the variable drive and the output shaft of the variable drive. While the Woods' device does disclose an automatic circuit for maintaining the desired speed of the output signal, there is no dead band adjustment in the Woods' drive system.